The present invention relates to an apparatus for imaging an object, especially a three-dimensional object, comprising first and second scanners each associated with a respective mirror for scanning the object in two directions, particularly two orthogonal directions.
An apparatus of this type is disclosed in PCT Application No. WO 90/00025 and contains a light source which can be focused on the object to be imaged. The focus of the light beam can be moved three-dimensionally over the object by means of a scanning system, and the light reflected from the object at each point can be recorded by means of a detection system. Two scanners or beam deflecting elements are provided for scanning in two directions. An arrangement of at least two imaging optical elements is provided in the beam path between the two beam deflecting means. This arrangement of optical elements is adjustable in order to change the horizontal deflection angle. Such optical elements between the two beam deflecting elements require a considerable amount of space. Regardless of the particular configuration, additional optical elements of this kind are basically required for optically perfect scanning if the scanning is performed in two directions by means of two beam-deflecting elements. For convenience, such beam-deflecting elements will be referred to hereinafter as "scanners." For the sake of completeness it will be noted that bidirectional scanning might also be performed by means of a scanner that can be moved in two axes. This, however, cannot be done at high scanning speeds, if the rapid scanning is performed, for example, by a resonant scanner or a polygonal mirror.
U.S. Pat. No. 4,318,582 discloses an apparatus for bidimensional scanning comprising a first polygonal mirror. The light which strikes and is reflected by the first polygonal mirror passes through a lens system to a second mirror, which is likewise rotatable or can turn about an axis perpendicular to the axis of rotation of the polygonal mirror. The light rays are reflected by this second mirror onto an object and reflected again by the object. The object is disposed in a plane which is orthogonal to the axis of the first polygonal mirror, and the axis of the second mirror is aligned substantially parallel to the plane. The two mirrors are synchronized by means of appropriate processors and control systems in order to scan the planar object line by line in two mutually perpendicular directions. Since the two centers of rotation of the scanning movements in the two directions are spatially separated from one another, a light beam reflected by the first mirror will not always strike the second mirror at precisely the same point. Therefore a lens system is needed between the two rotating mirrors, which images the first mirror on the second mirror, in order to reduce the movement of the light beam on the second mirror. Aside from the fact that the lens system increases the manufacturing cost and the size of the system, the lens system itself leads to imaging errors. The scanning system must be comparatively large in size, especially when the scanning angles are relatively large.
Finally, DDR Patent No. DD 266,654 discloses an optical scanning system which has two mirrors which cooperate upon the illumination of an object or when light rays reflected or emitted by the object are detected. One of these mirrors is positioned near the pupil of the apparatus. The other mirror is disposed at a point for aiming the light at the first mirror or at a point for receiving light which is reflected or emitted from the first mirror. This system can be configured as a scanning apparatus in which the first mirror rotates about an axis perpendicular to the axis of symmetry of the apparatus. The axis of the second mirror is perpendicular to that of the first mirror, the distance between the intersecting axes being of the same magnitude as the average distance between the centers of the two mirrors. No optical lens system is provided between the two mirrors, yet the movement of the light beams on the two mirrors requires a corresponding enlargement of the mirror. Above all, relatively large movements of the light beam on the first mirror cause disadvantageous imaging errors. The errors caused by the movement of the light beam on the mirrors adversely affect the scanning process, inasmuch as they increase with increasing movement and scanning angles.
There remains a need for an optical scanning apparatus which overcomes the foregoing disadvantages.